Some types of analog-to-digital converters convert analog signals to digital signals using a voltage controlled oscillator. In such converters, generally an analog input voltage signal is fed to the VCO, and the digital output value is determined based on the output of the VCO, which may for example involve a counting of pulses output by the VCO.
The resolution of such analog-to-digital converters is limited by non-linearities of the VCO. To improve the resolution, analog feedback or digital calibration are sometimes used. Analog feedback uses a large silicon area and increases the power consumption. The reduction of the non-linearity by digital calibration may be not sufficient for some applications.